The present invention relates to the sublimation growth of large single polytype crystals of silicon carbide.
Silicon carbide (SiC) has been known for many years to have excellent physical and electronic properties which theoretically allow production of electronic devices that can operate at higher temperatures, higher power and higher frequency than devices produced from silicon or gallium arsenide. The high electric breakdown field of about 4×106 V/cm, high saturated electron drift velocity of about 2.0×107 cm/sec and high thermal conductivity of about 4.9 W/cm-K make SiC conceptually suitable for high frequency, high power applications.
Silicon carbide also has an extremely wide band gap (e.g., 3 electron volts (eV) for alpha SiC at 300K as compared to 1.12 eV for Si and 1.42 for GaAs), has a high electron mobility, is physically very hard, and has outstanding thermal stability, particularly as compared to other semiconductor materials. For example, silicon has a melting point of 1415° C. (GaAs is 1238° C.), while silicon carbide typically will not begin to disassociate in significant amounts until temperatures reach at least about 2000° C. As another factor, silicon carbide can be fashioned either as a semiconducting material or a semi-insulating material.
As obstacles to its commercialization, however, silicon carbide requires high process temperatures for otherwise ordinary techniques, good starting materials are difficult to obtain, certain doping techniques have traditionally been difficult to accomplish, and perhaps most importantly, silicon carbide crystallizes in over 150 polytypes, many of which are separated by very small thermodynamic differences. Nevertheless, recent advances, including those discussed in U.S. Pat. Nos. 4,865,685 and 4,866,005 (now Re. 34,861) have made it possible to produce silicon carbide and silicon carbide based devices on a commercial basis and scale.
One of these advances has been the use of “off-axis” growth techniques to produce single (i.e., single polytype) crystal epitaxial layers and bulk single crystals. Generally speaking, the term “off-axis” is used to describe crystal growth techniques in which the seed crystal is presented for growth with its surface cut at an angle (usually between about 1 and 10 degrees) away from a basal plane or a major axis. Basically, it is accepted that an off-axis growth surface presents a large number of opportunities for step (lateral-step) growth that is controlled by the polytype of the crystal. The off-axis preparation of the substrate surfaces causes a series of steps and ledges to be formed to accommodate the (intentional) misorientations. The average spacing and height of the steps are largely determined by the degree of misorientation; i.e. the selected off-axis angle.
Alternatively, on-axis growth has been generally disfavored because an on-axis growth surface defines and presents far fewer ordered growth sites, and because the (0001) plane of silicon carbide does not contain polytype information. Instead, on-axis growth must proceed from defects and is thus less easily controlled.
Off-axis growth, however, presents its own unique challenges. In particular, bulk growth of large silicon carbide crystals (e.g. those suitable for wafer and device substrates) is generally carried out in seeded sublimation growth techniques, the details of which are laid out in the U.S. Pat. No. 4,866,005 incorporated above, and which have become familiar to those of skill in this art. Another relevant summary is set forth in Zetterling, Process Technology for Silicon Carbide Devices, INSPEC (2002) §§ 2.2.2.1-2.2.2.4.
In brief summary, a seeded sublimation technique includes a graphite (or similar) crucible that holds the source powder and the seed crystal. The crucible is heated (e.g. inductively) in a manner that establishes a thermal gradient between the source powder (hotter) and the seed crystal (cooler). The main gradient is typically referred to as the “axial” gradient because it typically (and intentionally) falls parallel to the axis of the growth crucible in which the source materials are at the bottom or lower portions and the seed crystal is attached to the upper portions and opposite some or all of the source materials. When the source powder is heated above SiC's sublimation temperature, the powder generates vaporized species that migrate generally or predominantly along the axial gradient and condense upon the seed crystal to produce the desired growth. In typical SiC seeded sublimation techniques, the source powder is heated to above about 2000° C.
During seeded sublimation growth of bulk single crystals, in order to keep the basal plane of the crystal (the plane that is parallel to the principal plane of symmetry of the crystal) on the growth surface of the crystal, off-axis techniques force the growth of convex crystals. In turn, encouraging such convex growth in a seeded sublimation technique typically requires the application of relatively high radial thermal gradients in addition to the axial (growth direction) thermal gradient. These radial gradients add stress to the growing crystal at growth temperatures, and these in turn create other stresses when the crystal cools to use or room temperature.
In silicon carbide, off-axis growth presents yet another problem based on the combination of two factors. As the first factor, in silicon carbide, the primary slip plane is the basal plane. As the second factor, changes in thermal gradients apply stress to crystals in the direction in which the gradient is changing; i.e., changes in the axial gradient apply stress to the crystal in the axial direction. As is understood by those familiar with such factors, the existence of thermal gradients does not generate the stress, but rather the rate of change of those thermal gradients. Stated differently, if a thermal gradient represents the change in temperature per unit of distance, the stresses are generated by the rate of change of the gradient per unit distance (i.e., a second derivative function).
When—as in off-axis sublimation growth—the basal plane is off axis to the major growth direction, these axial forces (caused by the rate of change of the gradient) include and thus apply a component parallel to the slip plane and thus generate and encourage slip defects. The magnitude of the basal-parallel component increases as the off axis angle increases in a manner common to the well-understood resolution of vectors into their respective components.
As a result, increasing the angle of the off-axis presentation of a growing silicon carbide crystal, or its seed, increases the slip forces applied to the basal plane and these forces in turn increase the concentration of slip defects per unit area.
By comparison, in on-axis growth the axial forces caused by changes in thermal gradients neither include nor apply a component parallel to the basal (slip) plane. Thus, slip defects can be more easily avoided during on-axis growth. As noted above, however, on-axis growth presents less polytype information and lower surface step density per unit area, thus making on-axis growth a relatively difficult technique for replicating the polytype of the seed crystal into the growing crystal.
Accordingly, both on and off-axis seeded sublimation growth techniques for silicon carbide present particular disadvantages.